1. Field of the Invention
The present invention relates to an eyepiece, and more particularly to a viewfinder lens system for use in a single lens reflex camera.
2. Description of the Prior Art
As a viewfinder lens system for use in a single lens reflex camera, a doublet type lens system which consists of two glass lens elements cemented to each other is known. However, such a construction requires a cementing step for cementing two lens elements to each other in its manufacturing procedure. Thus, it causes the increase of the cost of the production, the undesirable mixture of dust and bubbles in the cementing layer, the error of centration of two lens elements cemented to each other and, the undesirable coloring of the lens system due to the coloring of the cementing layer. Furthermore, such cementing may cause and increase of reflectance at the cemented surface, since a cementing material having a suitable refractive index for reducing the reflectance at the cemented surface does not exist.
A viewfinder lens system of a separate type which consists of two lens element with an air space therebetween is proposed. For example, in Japanese Laid-Open Patent Application (Tokkaisho) No. 53-135657 and U.S. Pat. No. 4,265,529, a viewfinder lens system, consisting from the object side, a positive first lens element and a negative second lens element with an air space formed therebetween, is disclosed. In this lens system, since the surface configuration of each of the elements is almost unchanged from the doublet type lens system, the radius of curvature of the eye side surface of the first lens element is almost equal to that of the object side surface of the second lens element. However, although the error of centration and the deviation of the thickness of the cemented layer from the designed thickness scarcely influences aberrations of the whole lens system in the doublet type lens system, in the separate type lens system, the deviation of the actual distance of two lens elements from the designed distance and the slight displacement of each lens elements will cause to be introduce a great change in aberrations.
In U.S. Pat. Nos. 4,217,048 and 4,437,750, another separate type viewfinder lens system which consists of, from the object side, a nagative first lens element and a positive second lens element with an air space formed therebetween is disclosed. However, in a lens system, since the radius of curvature of the eye side surface of the first lens element is almost equal to that of the object side surface of the second lens element and since such a radius of curvature is extremely small, any error in mounting would cause a great amount of aberration.